1. Technical Field
The present invention relates in general to bit patterned magnetic media and, in particular, to an improved system, method and apparatus for grading defective bit patterned magnetic media by disk sector in hard disk drives.
2. Description of the Related Art
The process of manufacturing a master disk for bit patterned magnetic media is expensive and time consuming. The projected cost of a master patterned media disk is more than one million dollars. One inherent difficulty in the manufacturing process for fabricating the master is the requirement that a rotary e-beam tool must run for several weeks to complete the master disk, Even with this careful and very precise process, there is a high probability that not all of the tracks on the master disk will be perfectly centered on the disk, or that the manufacturing process will form a perfect single domain, pre-defined, magnetic bit island.
For example, as illustrated in the schematic diagrams of FIGS. 1 and 2, a patterned media master disk 11 typically has many concentric or “on-center” tracks 13, 15 (e.g., two shown) of bit patterned media and, potentially, some non-concentric or “off-center” tracks 17 (e.g., one shown) of bit patterned media. Each track 13, 15, 17 comprises a circular array of discrete bits or islands 19. The intersection between the on-center track 13 and the off-center track 17 in Sector A causes two or more previously distinct islands 19 on the tracks to form one larger amalgamated bit 21 (FIG. 2). Furthermore, a sector on the disk that contains amalgamated bits 17 likely has a mirror-image amalgamated bit 21 on the other side of the disk (i.e., compare Sectors A and A′, which are about 180 degrees apart), thereby doubling the number of defective sectors.
One solution to this problem is to map the defective sectors on the master disk as being unusable. This solution is readily workable, but at a cost of reduced capacity of the disk. An alternate solution is to create a new master without the defective tracks, but at a significant additional cost. Although these solutions are workable, an improved solution that overcomes the problem and the limitations of prior solutions would be desirable.